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On Jan. 6, North Korea’s communist regime triggered what it claimed was a hydrogen bomb—the regime’s first—at an underground nuclear explosion at its Punggye-ri test site in the northern mountains.

The U.S. government has cast doubt on the H-bomb claim, pointing out that the blast better matches the profile of a much less powerful tritium-boosted fission device. In any event, the atomic test had the desired effect: “What happened there has definitely got our attention,” said U.S. Navy Vice Adm. Joseph Aucoin, commander of the 7th Fleet in the Pacific.

But two can play the nuclear brinkmanship game. The U.S. responded with a martial display of its own, starring a stalwart of America’s own huge nuclear arsenal.

On Jan. 10, a U.S. Air Force B-52—an eight-engine, jet-powered behemoth with a crew of five and a payload of up to 35 tons of bombs and missiles including nuclear munitions—flew an impressive 4,000-mile round trip to the Korean Peninsula from America’s main Pacific air base on the island of Guam. The B-52 met up with an escort of U.S. and South Korean jet fighters and performed a noisy, smoky, low-level pass over the peninsula within full view of the media’s cameras… and Pyongyang’s radars.

The provocative air show is just the latest example of what the Pentagon calls “signaling.” Or, in plain language, “bomber diplomacy.”

When a crisis flares, one of Washington’s first instincts is launch B-52s, swing-wing B-1s or stealthy B-2s as a show of force—a reminder to America’s allies and enemies alike that, despite mounting competition, America still possesses the world’s most fearsome military, including more than 150 bombers capable of striking almost anywhere in the world with just a few hours’ notice.

“The flights are the U.S.’s way of saying, ‘We know what you’re doing, we’re aware of your actions and we’ve taken note.’ You should take note of how we could, should we feel the need, respond,” Brian Laslie, an Air Force historian and author of The Air Force Way of War, told The Daily Beast.

Using aerial bombers to signal strength is not a new concept. Once bombers evolved into the globe-spanning, potentially world-destroying weapons that they are today, they inherited a deterrent potential that once belonged exclusively to battleships. In the early ’50s, Air Force Gen. Laurence Kuter, an early commander of the North American Air Defense Command (NORAD), said airpower should be used “to influence the behavior of other nations by actions short of war in support of national policy.”

For the next 40 years, whenever a major crisis flared involving the United States or Russia, bombers took to the air, not necessarily to drop any bombs, but as a warning that they could.

Signaling fell out of favor in the United States in the years after the Soviet Union’s 1991 collapse. But the rise of China and Russia’s resurgence under Vladimir Putin brought the practice back into vogue. In recent years, upgraded Russian bombers have probed U.S. and NATO airspace. China has sent its own new H-6K bombers into the Western Pacific, essentially announcing its newfound ability to exert military force over disputed territories in the East and South China Seas. In response, the Pentagon has flown B-52s over the same disputed territories.

Bomber diplomacy is enjoying a sort of renaissance. But there’s a problem for U.S. military planners. While the demand for bombers is increasing, supply in a sense is decreasing. America possesses the world’s biggest force of long-range bombers, but only the 20 stealthy B-2s can reliably penetrate the heaviest enemy defenses. The aging B-52s, which date to the early ’60s, and only somewhat less old B-1s from the ’80s lose some of their signaling potential as they become less and less useful during an actual shooting war.

Hence the Pentagon’s eagerness to acquire new stealth bombers. In 2013, the Air Force commissioned Forrest Morgan, an analyst at the RAND Corporation, a California think tank, to confirm whether bombers were still the best means of signaling military intent. Morgan’s answer was a resounding yes.

“Bombers generate a potent deterrent threat,” Morgan wrote, “without exposing U.S. forces to an inordinate amount of vulnerability to surprise attack,” because they can fly long distances from bases too far away for most enemies to bombard.

Reassured by Morgan’s assessment, in October 2015 the Pentagon awarded Northrop Grumman a potentially $79 billion contract to build as many as 100 long range strike bombers that the military expects to at least match the B-2’s ability to avoid detection—and to amplify the political signal that B-52s and other nuclear-capable bombers broadcast when they fly low and loud near enemy territory.

After all, no one expects North Korea to abandon its efforts to develop increasingly powerful nukes, nor Russia and China to roll back their expansionist foreign policy. Bomber flights could only grow in importance as the United States strives to peacefully assert its strength in an increasingly hostile world. “Bottom line, it’s about sending a very clear message,” Laslie said.